The present invention relates to a foamable resin composition that is useful for manufacturing metal castings and to a thermoplastic foamable pattern and metal casting manufacturing method using this composition.
Patterns of foamed thermoplastic material such as foamed polystyrene are used in full mold processes or eliminated pattern processes (also called `lost foam processes`) in casting of molten metal (also called `a melt`). One or more patterns or a mold runner system in correspondence to manufactured metal castings and a runner (sprue) are coated with refractory coating material (also called `mold facing`) and surrounded by sand in a flask defining a mold. When molten metal is poured into this mold, the patterns are decomposed and replaced by the metal and castings with the shapes of the patterns are produced through solidification.
Normally, a foamed polystyrene pattern used in this process is produced by introducing polystyrene beads that contain a readily volatizable foaming agent such as pentane and have been prefoamed by being heated and matured into a mold made of an ordinary metal such as aluminium and then heating the mold in order to cause the beads to fuse together and further causing the beads to foam so as to form the pattern. Generally, this secondary heating and foaming in the mold is effected by blowing steam into the prefoamed beads that have been packed in the mold.
It is well known that when a foamed polystyrene pattern is used in a full mold process or similar process, incomplete elimination or decomposition of the pattern can result in defects in the casting that is produced. Defects called `carbon defects` are the most marked in iron casting, and there is occurrence of wrinkles, depression at the time of cooling, surface holes and inclusions which are thought to be due to the presence in the mold of tar-like products resulting from thermal degradation of the polystyrene. A number of proposals such as for the use of additives in patterns have been made with the object of mitigating the problem of carbon defects relating to foamed polystyrene patterns. Japanese Unexamined Patent Publication No. 24146/1965 discloses the use of ammonium perchlorate as an additive for a pattern, Japanese Unexamined Patent Publication No. 16925/1966 discloses the use of a depolymerization catalyst and UK Unexamined Patent Publication No. 2193666A discloses the use of an oxidizing agent such as iron oxide or manganese dioxide which oxidizes carbon to carbon dioxide and/or carbon monoxide.
However, these proposals have the drawback that making the molds is quite difficult and the effects in overcoming carbon defects are not proportionally great.
It has further been proposed to use polymers other than polystyrene for foamed plastic patterns. U.S. Pat. Nos. 4,790,367 and 4,983,640 disclose the use of patterns made of polyalkyl acrylates such as polymethyl methacrylate. However, when a polymethyl methacrylate polymer is used, it is not possible to effect through impregnation with a foaming agent and the expansion ratio of the foamed grains is low. There is also the drawback that the moldability is inferior and the mechanical strength and the quality of the surfaces of molded products are poor. During polymethyl methacrylate casting, a large amount of gas is frequently emitted and this causes blow-back (splashing) and results in the casting having a poor surface.
There have been attempts to make patterns out of a mixture of prefoamed polystyrene and prefoamed polymethyl methacrylate but a number of problems arise in the use of these products or similar patterns. These are that it is necessary to prefoam the two substances and in mixing them it is difficult to produce a uniform mixture. Since static electricity makes filling of a mold difficult and the optimum conditions of pressure and temperature for molding polystyrene differ from the optimum conditions of pressure and temperature for molding polymethyl methacrylate, it is difficult to find optimum conditions for the two and a compromise is necessary. The resulting pattern is weak, since its density is uneven, and what happens is that the polystyrene and polymethyl methacrylate beads only fuse with themselves and not with one another. In addition, the pattern has a poor surface.
Japanese Patent Publication No. 23458/1974 discloses a foamed pattern in which isobutyl methacrylate, which has good thermal decomposition characteristics, is made the main component and methyl methacrylate is used as a copolymerization component and also proposes a foamed pattern which has methyl methacrylate as the main component.
The abovenoted foamed patterns for which isobutyl methacrylate is used as the main component and methyl methacrylate is used as a copolymerization component have the drawback that their glass transition temperature is very much lower than that of polystyrene foamed bodies and if primary foaming and molding equipment that are used for polystyrene foamed bodies are employed there is considerable shrinkage of primary foamed grains and molded products. It is therefore necessary to use different molding units and to alter the molding conditions such as, e.g., the temperature and pressure, etc.
Because of this, a method of copolymerizing 80-95% by weight of methyl methacrylate and 20-5% by weight of styrene was proposed in the disclosure of Japanese Patent Publication No. 24307/1976 with the object of improving foaming characteristics. Also, a method of effecting copolymerization with addition of .alpha.-methylstyrene as a supplementary component at the time of methyl methacrylate polymerization was proposed in the disclosure of Japanese Patent Publication No. 40160/1975. Further, the use of a methyl methacrylate and .alpha.-methylstyrene copolymer foamed pattern is described in the disclosure of Japanese Patent Publication No. 184447/1985.
However, even with these foamed patterns for which methyl methacrylate is made the main component and styrene or .alpha.-methylstyrene is used as a copolymerization component, there is still the drawback of considerable shrinkage of primary foamed grains and of molded products and it is difficult to simultaneously achieve an increase in the primary expansion ratio and a reduction of shrinkage at the time of molding. Further, with patterns such as these there tends to be emission of an excessive amount of gas, so resulting in the production of castings with inferior surfaces.